1. Field of the Invention:
The present invention relates to a liquid crystal (LC) display apparatus. More particularly, the present invention relates to an LC display apparatus incorporating an LC display device with controllable viewing angle characteristics. The present invention also relates to an LC display apparatus in which highly-collimated light is emitted toward an LC display device thereof.
2. Description of the Related Art:
LC display apparatuses are widely employed as digit segment type display devices for clocks and calculators and as display devices for displaying general images. A number of types of LC display devices to be incorporated in LC display apparatuses are known. For example, one type of LC display device includes active elements, e.g., TFTs (thin film transistors), provided on a light-transmitting substrate, the active elements functioning as switching means for selecting pixel electrodes for applying a voltage to liquid crystal. Another type of LC display device is characterized by including color filter layers of red, green, blue, or the like are provided as color display means.
Since LC display devices consume relatively little power, they are often employed as display devices for mobile or portable appliances, e.g., laptop-type personal computers and word processors. An LC display device can adopt any one of various display modes. Examples of display modes which utilize electrooptical effects include a TN (twisted nematic) mode, an STN (super twisted nematic) mode, an ECB (electrically controlled birefringence) mode, and an OCB (optically compensated birefringence) mode.
An active driving type TN liquid crystal display mode includes liquid crystal molecules twisted at a twist angle of 90.degree.. A multiplex driving type STN liquid crystal display mode includes liquid crystal molecules which are twisted at a twist angle of 90.degree. or more so as to utilize the steepness transmittance/voltage (applied to the liquid crystal) characteristics.
TN type LC display devices and STN type LC display devices are known to have display characteristics problems that are inherent within their display principles. Specifically, in an LC display device of either a TN or STN mode, the angle or direction in which liquid crystal molecules can be observed when a voltage is applied is uniquely determined by the ideal viewing angle prescribed for each device. Thus, as the viewing angle is tilted with respect to the orthogonal direction (which defines the ideal viewing angle), the light transmittance varies accordingly. This phenomenon is especially marked in gray scale tones. Therefore, the acceptable range of viewing angles is considerably limited in the case of the TN and STN modes.
Conventional techniques for improving the viewing angle characteristics of such LC display devices have been proposed. One technique alters the orientation state of liquid crystal in the cell. Another technique broadens the extent of the light on the viewer's side (as controlled by an illumination system) by utilizing the frontal (orthogonal) characteristics of a TN type LC display device. There has also been proposed a polymer dispersed type LC display device, which performs a display function by controlling the transmitted light based on light scattering control. This method provides relatively broad viewing angle characteristics in the case of a scatter type LC display device because the contrast of the display screen does not depend on the relationship between the orientation direction of the liquid crystal molecules and the direction of observation.
First, the conventional technique for improving viewing angle characteristics by altering the orientation state of the liquid crystal molecules in the cell is described. In order to improve the viewing angle characteristics of an LC display device incorporating polarizing plates by this method, it is necessary to ensure that the liquid crystal molecules are oriented in at least two different directions within each pixel.
FIG. 1A schematically shows liquid crystal molecules being oriented in two directions within a region of a LC layer that corresponds to each pixel. While displaying a gray scale tone in this orientation state, the transmittance (refractive index) when the LC layer is viewed in direction 1a and that when the LC layer is viewed in direction 1b are averaged to become substantially equal. As a result, the viewing angle characteristics are improved as compared with the characteristics of the conventional TN mode illustrated in FIG. 1B. In FIG. 1B, the liquid crystal molecules are tilted in the same direction when displaying a gray scale tone, so that the apparent refractive index differs depending on whether the LC display device is viewed in direction 1a or direction 1b. This results in a large difference in contrast depending on whether the LC display device is viewed in direction 1a or direction 1b. Consequently, display problems such as an inversion of white/black or hue may occur.
Specific examples of attempts to obtain broad viewing angle characteristics include the following.
First, examples of attempts to improve the viewing angle characteristics based on the structure inside the LC display device (LC layer) are described in the following paragraphs (numbered (1)-(6)):
(1) A method has been proposed for electrically controlling the transparent state and the white state of liquid crystal by utilizing the birefringence of the liquid crystal. An LC layer includes a liquid crystal region and a support medium (polymer wall). The LC display device does not require any polarizing plates or alignment treatments. In principle, this method ensures that the ordinary light refractive index of the liquid crystal molecules coincides with the refractive index of the support medium so that a transparent state is displayed when a voltage is applied to align the liquid crystal molecules and a scattering state is displayed when no voltage is applied so as to allow the liquid crystal molecules to be randomly oriented (scattered type).
As an example of this method, Japanese laid open National Patent Publication No.61-502128, for example, discloses a method in which a liquid crystal and a photocurable or thermocurable resin are mixed together and the resin is thereafter cured by the application of light or heat so that liquid crystal regions are deposited, thereby forming liquid crystal droplets within the resin.
Furthermore, Japanese Laid-open Patent Publications Nos. 4-338923 and 4-212928 disclose techniques concerning a broad viewing angle mode LC display device which incorporates the above-mentioned LC display device along with polarizers.
(2) A method for improving the viewing angle characteristics of a non-scattering type LC display device using a polarizer is disclosed in Japanese Laid-open Patent Publication No. 5-27242. This method subjects a mixture of the liquid crystal and a photocurable resin to a phase separation to obtain a composite material of a liquid crystal and a polymer. According to this method, the orientation state of the liquid crystal molecules in each liquid crystal region becomes random due to the produced polymer disturbing the orientation of the liquid crystal molecules. Therefore, the liquid crystal molecules stand in different directions in each minute domain in each liquid crystal region including randomly-oriented liquid crystal molecules, so that the average value of the transmittance in that state becomes substantially identical regardless of the viewing direction. As a result, the viewing angle characteristics when displaying a gray scale tone are improved.
(3) Recently, the inventors of the present invention have proposed a method for orienting liquid crystal molecules in an omnidirectional manner (e.g., in a spiral shape) within each pixel region by controlling the intensity of light irradiated for photopolymerizing a mixture including a liquid crystal and a photopolymerizable resin by means of a photomask or the like (Japanese Laid-Open Patent Publication No.7-120728). According to this method, the liquid crystal molecules are voltage-controlled so that the spiral orientation shifts to a homeotropic orientation without ruining its omnidirectionality. As a result, the angle dependence of display is alleviated, whereby the viewing angle characteristics of the LC display device are remarkably improved.
(4) The inventors of the present invention have also proposed an LC display device in Japanese Laid-Open Patent Publication No.6-308496, which incorporates a crystalline polymer film having spherulites formed therein as an alignment film. As a result, the liquid crystal molecules are axially symmetrically oriented so as to realize a broad viewing angle characteristics mode. As used herein, a spherulite is defined as a spherical polycrystal in which a number of minute crystals (crystallets) are radially aligned with respect to a center.
(5) Japanese Laid-Open Patent Publication No.6-194655 discloses a method for randomly orienting liquid crystal molecules by applying an alignment film on a substrate without performing any alignment treatments (e.g., rubbing) for the alignment film.
(6) Japanese Laid-Open Patent Publication No.57-186735 discloses a method for aligning the liquid crystal molecules in a plurality of regions into which each pixel is divided so that the orientation states of the respective divided regions compensate for one another to realize broad viewing characteristics.
Furthermore, Japanese Laid-Open Patent Publication No.5-72529 discloses an LC display device utilizing the optical rotatory power and birefringence of a TN liquid crystal or the like which includes an LC layer for controlling the light scattering, as well as an apparatus and a system incorporating the LC display device. The luminance, hue, and contrast ratio of the LC display device are disclosed as having a small viewing angle dependence. This LC display device attains viewing angle characteristics similar to the viewing angle characteristics of the conventional TN liquid crystal when a voltage is applied to the additional LC layer so as to attenuate light scattering (i.e., a light transmitting state), and hence the LC display apparatus becomes difficult to be seen by anyone other than the user thereof.
Next, an example of an attempt to improve the viewing characteristics by controlling light in an illumination system and on the viewer's side, rather than based on the structure inside the LC display device (LC layer) is described.
SID 95 DIGEST, 36.2 (p.793-796) reports a method for collimating light which enters an LC display device and further broadening the extent of light coming out through the front face of the LC display device on the viewer's side. According to this method, light entering the LC cell is substantially collimated by using an optical filter for controlling the direction of the travel of light (e.g., a directional optical filter disclosed in U.S. Pat. No. 4,621,898) or the like. Furthermore, a sheet capable of diffusing light is provided on the viewer's side (i.e., the side through which the light comes out of the LC cell) of the display device. According to this method, the incident light substantially vertically travels through the LC display device (e.g., the TN mode or the STN mode), so that a high contrast is obtained and the color shift is prevented. Furthermore, by adding a diffusion sheet on the side of the LC cell opposite the light source, the frontal characteristics of the LC display device can be appreciated over a broad range of viewing angles.
The above-mentioned appliances (e.g., personal computers and word processors) incorporating LC display devices are beginning to be used for various purposes and in various situations. For example, in the case where a presentation meeting is held by using such appliances, it is desirable that the display device have broad viewing angle characteristics because a number of people will simultaneously watch the display device. On the other hand, in the case where such appliances are used for inputting and displaying information in public places or in public transportation means, e.g., airplanes or trains, it is desirable that the viewing angle characteristics be just broad enough for the user to be comfortable so that others cannot peek at the display screen, out of privacy and confidentiality concerns. Such different needs have required a single apparatus that includes a display device having controllable viewing angle characteristics so that the display device can be adapted from a broad viewing angle mode (for a number of simultaneous users) to a narrow viewing angle mode (for one user only).
However, the above-mentioned conventional LC display devices, which attempt to broaden the viewing angle characteristics, cannot adapt the viewing angle characteristics to different needs, especially in an electrical manner.
Moreover, the LC display apparatus disclosed in Japanese Laid-Open Patent Publication No.5-72529 has the following problem. The directionality of backlight provided in a conventional display apparatus is such that the light expands in directions other than the LC display device itself so that it can be observed by people who are not orthogonally situated with respect to the display screen thereof. That is, in accordance with a conventional structure where lowly-collimated light is emitted toward the LC display device, the viewing angle characteristics of the LC display apparatus cannot be made sufficiently narrow so that only a person who is situated orthogonally with respect to the display screen can observe the displayed image, even by placing the additional LC layer for controlling the light scattering in a light transmitting state in order to allow little light to be scattered. Therefore, in order to ensure that only a person who is situated orthogonally with respect to the display screen can observe the displayed image in the above-mentioned LC display apparatus, it is necessary to emit highly-collimated light toward the LC display device.
LC display apparatuses having other structures may similarly require highly-collimated light to be emitted toward the LC display devices.